P836: ANTIBODY-DRUG CONJUGATES AS POTENTIAL CAR-T CELL SPECIFIC OFF-SWITCH IN MULTIPLE MYELOMA
Authors: Anna Bielowski, Teresa Kilian, Sarah Rümpeler Calheiros Vera-Cruz, Zeno Riester, Sabrina Prommersberger, Julia Beckmann, Martin Kortüm, Leo Rasche, Hermann Einsele, Michael Hudecek, Sophia Danhof
Journal: HemaSphere (2023)

Topic: 13. Myeloma and other monoclonal gammopathies - Biology & Translational Research Background: The recent approval of BCMA-targeting CAR-T cell products for refractory/relapsed multiple myeloma (MM) has reframed the landscape of MM therapy by showcasing impressive clinical responses. However, common limitations include early relapses, and alternative targets for CAR-T cell therapy in MM are currently under investigation. First-in-human applications of novel CAR-T cell products pose the risk of severe side effects like cytokine release syndrome, neurotoxicity and on-target/off-tumor toxicities. Aims: The incorporation of efficient suicide strategies for CAR-T cell depletion are therefore of highest relevance. In this regard, monoclonal antibodies (mAB) directed against CAR-T cell transduction markers have been suggested. However, we postulate that severe lymphodepletion can compromise their CAR-T cell elimination capacity by reducing antibody-dependent cell cytotoxicity (ADCC) and we suggest alternative strategies. Methods: We generated CD19 and SLAMF7 CAR-T cells with truncated EGFR (EGFRt) or BCMA as transduction markers. In an unspecific approach, we exposed the CAR-T cells to dasatinib, a potent inhibitor of intracellular CAR signaling, as well as to the lymphodepleting drugs fludarabine and the pre-activated cyclophosphamide analogue mafosfamide. Furthermore, for CAR-T cell specific depletion, we co-cultured CAR-T cells with PBMC, or subsets thereof, in the presence or absence of the anti-EGFR antibody cetuximab; or with the antibody-drug conjugate (ADC) belantamab mafodotin, a mAB against BCMA conjugated with the cytotoxic agent mafodotin. At defined time points, we evaluated absolute numbers of CAR-T cells and, where applicable, their effector functions. Results: First, we investigated the capacity of cetuximab to deplete EGFRt-positive CAR-T cells in vitro. Using PBMCs as effector cells for selective elimination of CAR-T cells with cetuximab had modest effects, however ADCC rates could be enhanced by enriching NK cells. Nevertheless, ADCC of SLAMF7 CAR-T cells remained significantly lower than ADCC of CD19 CAR-T cells. Of note, in a cohort of MM patients treated with SLAMF7 CAR-T cells (n=5), we found NK cell counts to be dramatically reduced between CAR-T cell infusion (day 0) and day +14. Besides the general pre-CAR-T cell conditioning therapy, NK cell depletion in this setting was likely resulting from CAR-T cell cytotoxicity towards SLAMF7-positive NK cells, as previously shown (Gogishvili T et al. Blood 2017). In an unselective approach to control T cells, we found dasatinib to reversibly impair CAR-T cell proliferation, cytokine secretion and target cell lysis in an antigen-specific manner, whereas fludarabine and mafosfamide depleted both, CAR-T cells and untransduced T cells in an unspecific manner. To maintain a CAR-T cell specific approach but circumvent the problem of impaired ADCC in the setting of NK cell deficiency, we co-expressed BCMA on our CAR-T cells before exposing them to belantamab mafodotin. This resulted in potent elimination of BCMA-positive CAR-T cells independent of effector cells, while untransduced T cells were spared. Summary/Conclusion: In aggregate, we demonstrate that CAR-T cell depletion strategies relying on a functional effector cell compartment might be of limited relevance during the time period of intense lymphodepletion and/or for cell products (co-)targeting NK cells. Besides unspecific T cell inhibition with dasatinib or depletion with fludarabine and mafosfamide, the use of ADCs has the potential to control CAR-T cell associated toxicities in a CAR specific manner independent of the patient’s immunological fitness. Keywords: Cancer immunotherapy, CAR-T, Multiple myeloma, Adoptive immunotherapy